We use flower development in Arabidopsis as a model to understand how cells in multicellular organisms assume their developmental fates and form distinct patterns. Our long-term goal is to identify and analyze the genetic networks and the underlying molecular interactions that lead to cell fate specification in the floral primordium. For a long time, three classes of floral homeotic transcription factors known as the A, B, and C genes have been the only players in floral organ identity specification. Our studies have identified a microRNA, miR172, as a posttranscriptional represser of the class A gene APETALA2, indicating that posttranscriptional regulation also plays a role in flower development. In the proposed project, we will employ molecular genetic approaches to determine how miR172 fits in the regulatory circuitry governing flower development. In particular, we wish to 1) establish the developmental roles of the five MIR172 genes by analyzing mutants or RNAi lines; 2) determine whether miR172 leads to the proper spatial restriction of AP2 expression in the floral primordium; 3) investigate whether the ABC genes contribute to the highly specific spatial patterns of miR172 in the floral primordium; and 4) test whether miR172 mediates the antagonistic interactions between A and C genes. Homeotic genes that encode transcription factors act in cell fate specification in both animals and plants. Emerging evidence of microRNAs as regulators of homeotic genes in both animals and plants adds a new layer of regulation to the known transcriptional networks governing cell fate specification. This proposed research will undoubtedly provide insights into the integration of posttranscriptional and transcriptional mechanisms in developmental processes in multicellular organisms. [unreadable] [unreadable]